Fischer's ideas informed the Nuremberg Laws of 1935 which served to justify the Nazi Party's belief in German racial superiority.[1]Adolf Hitler read Fischer's work while he was imprisoned in 1923 and he used Fischer's eugenical notions to support the ideal of a pure Aryan society in his manifesto, Mein Kampf (My Struggle).[1]

In 1906, Fischer conducted field research in German South West Africa (now Namibia). He studied the Basters, offspring of German or Boer men who had fathered children by the native women (Hottentots) in that area. His study concluded with a call to prevent a "mixed race" by the prohibition of "mixed marriage" such as those he had studied. It included unethical medical practices on the Herero and Namaqua people.[9] He argued that while the existing Mischling descendants of the mixed marriages might be useful for Germany, he recommended that they should not continue to reproduce. His recommendations were followed and by 1912 interracial marriage was prohibited throughout the German colonies.[10][11] As a precursor to his experiments on Jews in Nazi Germany, he collected bones and skulls for his studies, in part from medical experimentation on African prisoners of war in Namibia during the Herero and Namaqua Genocide.[12][13]

His ideas expressed in this work, related to maintaining the purity of races, influenced future German legislation on race, including the Nuremberg laws.[11]

In 1927, Fischer was a speaker at the 1927 World Population conference held in Geneva, Switzerland. The Conference was organized by the League of Nations and Margaret Sanger, the founder of The American Birth Control League which was later rechristened as Planned Parenthood.[14]

In the years of 1937–1938 Fischer and his colleagues analysed 600 children in Nazi Germany descending from French-African soldiers who occupied western areas of Germany after First World War; the children were subsequently subjected to sterilization afterwards.[15]

Fischer did not officially join the Nazi Party until 1940.[16] However, he was influential with National Socialists early on. Adolf Hitler read his two-volume work, Principles of Human Heredity and Race Hygiene (first published in 1921 and co-written by Erwin Baur and Fritz Lenz) while incarcerated in 1923 and used its ideas in Mein Kampf.[17] He also authored The Rehoboth Bastards and the Problem of Miscegenation among Humans (1913) (German: Die Rehobother Bastards und das Bastardierungsproblem beim Menschen), a field study which provided context for later racial debates, influenced German colonial legislation and provided scientific support for the Nuremberg laws.[18]

Under the Nazi regime, Fischer developed the physiological specifications used to determine racial origins and developed the so-called Fischer–Saller scale. He and his team experimented on Gypsies and African-Germans, taking blood and measuring skulls to find scientific validation for his theories.

Efforts to return the Namibian skulls taken by Fischer were started with an investigation by the University of Freiburg in 2011 and completed with the return of the skulls in March 2014.[19][20][21]

In 1944 Fischer intervened in an attempt to get his friend Martin Heidegger released from service in the Volkssturm militia. However, Heidegger had already been released from service when Fischer's letter arrived.[22]:332-3